How to change form background in Classic forms?

Yes thanks, but I found its look and feel became very simple.. like google
form :P
We dont havebeautiful background in all Questions-in-one page format?

You can add the background image to Classic Forms (All Questions-in-one page format) using the Advanced Designer.

https://www.jotform.com/form-designer/tuts/episode10/ 

This is the background image link of your form - https://www.jotform.com/uploads/card_form_themes/form_files/hoo.5a78ef19a8c6c6.48314239.jpeg 

Here is my demo form: https://form.jotform.com/81404473070954 

Feel free to clone it. 

Yes thank you but the look and feel totally changes.
I like the look and feel of the one post per page but the all in one page
post changed it totally
...

You can switch the form to Card layout and have a custom image background; see demo below:

You can switch the layout in the Form Settings.